PDCD1 and Autoimmunity: Engagement of PD-1 on T cells by its physiological ligands, programmed cell death protein 1 ligand 1 (PD-L1, also known as CD274, or B7-H1) or programmed cell death protein 1 ligand 2 (PD-L2, also known as CD273, or B7-DC), induces T cells into a dysfunctional state known as exhaustion, and thus serves as an immune checkpoint to prevent autoimmunity under physiological conditions [1,2,3,4,5].